Monday, March 25, 2013

Jazz Disassemblies Ep0: Me, Myself and the 8-bit Computer

Episode 0: Me, myself and the 8-bit computer

In both the video game and chiptune/modding scenes I am known me as Jazz or Jazzmarazz, but few know me by my real name; Jordan Appleton-Joslin. This past December, I finally graduated from Central Michigan University with a Bachelors of Science in Information Tech with a minor in Industrial Tech. Studying computer technology through the eyes of your everyday "end-user" simply didn't cut it. From very early on I was overly fascinated with electronics and what makes them work; while never being satisfied with just using them. I quickly finished Uni's requirements to graduate with IT and picked up a huge number of EE classes to fill in the rest of the gaps.

For as long as I remember the Nintendo and Atari took up the largest part of my childhood; playing games and pumping the audio through my father's stereo with unknowingly sketchy and possibly reckless hand-coiled wiring. While my brother tooled around with his walkman, I carried my gameboy listening to the sound files available through option menus. Naturally I found my way into the chip/mod crowd for future-learning-endeavors and old time's sake.

Each week following today will be accompanied by a dis-assembly of some sort. I plan to accept any and all gizmos and gadgets in the name of science! So please send your 70's, 80's and even early 90's crap so that I may break it down and tell you a little more about what makes it tick and or how it may be used for chip-relevant pursuits!

If you have something which piques your interest but cannot find a use, please contact me; broken or otherwise, so please hit up them yard sales and thrift stores! :D

Now that you know all about me and myself, lets move on to the main attraction. As today's special guest, I have not chosen a gameboy, C64 or Sinclair something or other...but today you will all learn about the Compumate2 from Laser! Originally purchased at an estate sale after the death of the prior owner, this wonderful device was thought to be the one and only Compumate2 for the Atari 2600; however, it is not. I sat this aside for several years once I found out that it was nothing more than a miserable PDA from the 80's.

Full QWERTY keyboard and 7 functions and I was still rather peeved having just obtained an Atari after many long years. Recently though, I had become very interested in the Zilog z80 and Intel 8085. For several weeks, I worked on building my very own 8-bit computer based on the 8085 and even though it was much more powerful; I still wanted to work with the z80. The z80 as some of you may know is what powers each and everyone of your gameboys. Sure, it has many proprietary modifications to interface with the link port, LCD and button input; but it is still a z80 at heart.

So I tore into the PDA not looking for anything fun, but looking for components to scrap. To my surprise, I found not only a z80 micro Processor, but 256Kbit ROM, 64kbit RAM, 2 x 20 LCD and possibility for external programs to be written all running on 4xAA's! Not a whole lot has been shared about this device and even less is known about the external ROM slot, but know this: I will eventually find out how to run a custom program on it. See below that there is a 34 pin connection at one side of the board. Also note the RAM chip being socketed. Laser must have had plans to expand the RAM at one point, but up to what capacity?

Also notice that the sides have empty ports for expansion. The labels read: Line, Phone, Cassette and Expansion I/O. There seems to be limited room inside to fit many more components, but looking at pictures of the Compumate3 which itself is incredibly rare, I found that the I/O port at least was fitted with a midi/game type connection. Could there have been plans to release games for use with commercial PC controllers of that generation? There is no information on any carts being released to support a sure answer. In any case, the computer is equipped with a speaker for simple 1 bit beeps to signal an error or process completion, so why not write a simple 1-channel tracker once the code is analyzed?

If you looked closely at the external ROM connector, it is nothing more than a 2 x 17 DIP connection....exactly that of a floppy drive cable used in last year's PC! Following the pins back to their origins, I find that the ROM slot is directly interfaced with the Address and data buses, but what else...34 pins and only 24 D/A pins...I guess you'll have to wait for the second installment! xP

Thanks for tuning in!

Cheers,
Jazz

p.s. I am including the hex dump and (hopefully reliable) disassembly of the internal ROM as well as various pinouts that I traced with my multimeter:Compumate2 ROM (BIN)Various Pinouts (TXT) (Best to be viewed in word pad rather than your browser.)
Use these only for good! ;)